This invention relates generally to a raster output scanning system for producing a high intensity imaging beam which scans across a movable photoconductive member to record electrostatic latent images thereon, and, more particularly, to an apparatus for providing controlled registration of the beam in the process direction movement of the photoconductive member.
In recent years, laser printers have been increasingly utilized to produce output copies from input video data representing original image Information. The printer uses a raster output scanner (ROS) to expose the charged portions of the photoconductive member to record the electrostatic latent image thereon. Generally, a raster output scanner has a laser for generating a collimated beam of monochromatic radiation. The laser beam is modulated in conformance with the image information. The modulated beam is reflected through a lens onto a scanning element, typically a rotating polygon having mirrored facets. The light beam is reflected from a facet and thereafter focused to a "spot" on the photosensitive member. The rotation of the polygon causes the spot to scan linearly across the photoconductive member in a fast scan (i.e., line scan) direction. Meanwhile, the photoconductive member is advanced relatively more slowly than the rate of the fast scan in a slow scan direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster scanning pattern. The light beam is intensity-modulated in accordance with an input image serial data stream at a rate such that individual picture elements ("pixels") of the image represented by the data stream are exposed on the photosensitive medium to form a latent image, which is then transferred to an appropriate image receiving medium such as paper.
Data in each of the fast and slow scan directions is generally sampled. The sampling rate of the slow scan direction data equates to 300 lines per inch or more In many printing apparatus. It has been shown that errors in the slow scan direction of as small as 1% of the nominal line spacing may be perceived in a half tone or continuous tone image. This implies a need for a high degree of control in positioning the spot in the slow scan direction on the image plane, especially in such applications as multiple beam and multiple ROS color printers where a plurality of spots are written onto a single photoreceptor. Furthermore, high resolution printing, on the order of 600 spots per inch or higher, demands very accurate spot positioning.
Errors of the spot position in the slow scan direction arise from many sources, including polygon and/or photosensitive member motion flaws, facet and/or image plane (e.g., photosensitive medium) surface defects, etc. These errors are most commonly addressed by passive or active in-line optics. Some prior art examples are disclosed in:
U.S. Pat. No. 4,600,837 to DiStefano et al. discloses an optical scanning apparatus with dynamic scan path control wherein a scan path is altered by two different prisms, which are positioned before a polygon in the scan path. The two prisms alter the scan path in both a horizontal and a vertical direction. The two prisms are controlled by error signals generated by a grating. A phase error resulting from a comparison of the grating signals with a system clock is shown in FIG. 5. See Col. 5, lines 41-66.
U.S. Pat. No. 4,660,094 to Yoshimoto et al. discloses a method of focus adjustment of a picture scanning and recording system wherein a projection lens is moved automatically in a direction normal to a recording drum to correct for drum variations caused by machining or environmental conditions, such as temperature. A rotary encoder generates pulses which are representative of the recording drum's rotation. The pulses are used by a motor to move a mirror assembly closer to or farther away from the drum. An image sensor, within the light path, detects when the beam is out of focus and adjusts it. See Col. 2, lines 31-64.
U.S. Pat. No. 4,040,096 to Starkweather discloses a basic polygon ROS structure having runout and/or facet errors (both scanning errors in the slow scan direction) by locating a cylindrical lens in the optical path, either pre- or post- polygon, which focuses the beam in the slow scan direction onto the desired focal plane.
U.S. Pat. No. 4,858,019 to Ohara et al. discloses a light scanning recording device wherein a recording medium's speed is determined by counting pulses generated by a reference clock signal. The pulses, which are inversely related to scanning speed, are fed back to control the speed of the recording medium. Another feed back signal is used to control polygon speed. See Col. 3, lines 43-55.
Also, relevant disclosures are contained in U.S. patent applications assigned to the same assignee as the present invention. Each of the contents of these applications are hereby incorporated by reference.
Each of these various prior art schemes have disadvantages or shortcomings. For example, the use of high quality optics requires not only high quality optical elements, but also tight control in the positioning of those optics, in order to obtain the requisite very precise mechanical control sufficient to adjust spot position 0.02 mm or less, required in many cases. In order to achieve this level of spot position control with the acousto-optic modulators, an acoustic wave must be established and maintained with great precision. These acousto-optic modulators are relatively quite expensive, and require an associated accurate high frequency signal generator and related electronics to produce and maintain the acoustic waves. Further, those systems which incorporate feedback circuits to move rotating mirrors or translating roof mirrors are generally too slow to correct for motion quality errors because these relatively bulky mirror components are difficult to move precisely and quickly.
According to the present invention, the ROS system includes a cylinder lens in the prepolygon optics to focus the beam in the slow scan direction onto the polygon facets. The cylinder lens, a relatively light optical component in the prepolygon optical path, is adapted to be moved in the plane parallel to the process direction plane, so as to correct the location of the scanned beams at the photoreceptor. The correction Is enabled by providing a phase error feedback circuit for generating error signals which are sent to a pizeo-electric actuator to provide high frequency control in the process direction of the position of the cylinder lens. The invention is especially intended to provide compensation for photoreceptor motion (vibration) errors in the range of approximately 0-150 hz.
More particularly the present invention relates to a light scanning apparatus comprising:
means for generating a laser beam,
means for modulating the amplitude of the beam in accordance with input video data,
a multi-faceted reflector polygon positioned in the beam path,
means for rotating the polygon, the beam being reflected from successive facets of the polygon and sweeping along a scan path to provide successive scan lines along the surface of the photoreceptor moving in the process direction, the improvement comprising a cylinder lens located between said laser generating means and said polygon, said cylinder lens focusing the output of said laser in the process direction, and
means for moving said cylinder lens in a plane parallel to the process direction to correct for scan line image motion quality errors occurring at the photoreceptor in the process direction.